Leap Year Checker

What does this Leap Year Checker do?

This free online leap year checker tells you instantly whether any given year is a leap year. It applies the exact rules of the Gregorian calendar — the same rules used by virtually every country in the world today — and shows you the step-by-step reasoning behind the answer, not just a yes or no.

You can check a single year, see the next and previous leap years, and scan an entire range to list every leap year between any two years. Each check walks through the three divisibility rules in order, so you can see exactly why a year like 1900 fails and why a year like 2000 passes.

Everything runs locally in your browser. No server, no signup, no tracking. Your input never leaves your device.

How to use the Leap Year Checker

1. Type a year: Enter any year between 1 and 9999 in the input field. The checker accepts years from the Gregorian calendar and proleptic Gregorian (years before 1582).
2. See the result instantly: The result appears as you type, with a clear "Leap Year" or "Not a Leap Year" verdict, plus a color-coded card so you can tell at a glance.
3. Read the rule breakdown: Below the result, a formula box shows each of the three divisibility checks in order, with pass/fail indicators, so you can see exactly how the answer was derived.
4. Try the quick examples: Click any of the preset year buttons (1900, 2000, 2023, 2024, 2100, 2400) to see how century years behave.
5. Scan a range: Use the range scanner to list every leap year between two years — perfect for planning or homework.
6. Copy or explore: Copy the result to your clipboard, or click "Next 5" / "Previous 5" to see the leap years around the one you checked.

Why use this Leap Year Checker?

The Gregorian leap year rules

A year is a leap year if it satisfies these three rules, applied in order:

1. Divisible by 4: If the year is not divisible by 4, it is not a leap year.
2. Not a century year: If the year is divisible by 100 (a century year), it is not a leap year — unless the next rule applies.
3. Divisible by 400: If the year is divisible by 400, it is a leap year, even if it's a century year.

The result: a leap year occurs every 4 years, except when a century year is not divisible by 400. That means in any 400-year cycle, there are exactly 97 leap years — not 100, as you might expect from "every 4 years."

Why do leap years exist?

A tropical year — the time it takes Earth to complete one orbit around the Sun — is approximately 365.2422 days. A calendar year of 365 days is therefore short by roughly 0.2422 days, or about 5 hours, 48 minutes, and 46 seconds every year.

If we ignored this drift, the calendar would slowly fall out of sync with the seasons. After 100 years, the calendar would be off by about 24 days — meaning January would feel like autumn in the Northern Hemisphere. After a few centuries, the error would be unmistakable.

Adding one extra day (February 29) every four years brings the average year length to 365.25 days, which is close to 365.2422 but still slightly too long. That leftover error adds up to roughly one day every 128 years — which is why the Gregorian calendar adds the century-year exception. Skipping three leap years every 400 years brings the average year to 365.2425 days, accurate to within about one day every 3,200 years.

The Julian calendar vs. the Gregorian calendar

The Julian calendar, introduced by Julius Caesar in 45 BC, used a simple rule: every 4th year is a leap year. This worked well for centuries, but the accumulated drift added up. By 1582, the calendar was about 10 days out of sync with the seasons.

Pope Gregory XIII introduced the Gregorian calendar in 1582 to fix this. It kept the every-4-years rule but added the century-year exception: century years are only leap years if divisible by 400. This corrected the drift and brought the calendar back in line with the seasons. The Gregorian calendar is now used — officially or unofficially — by virtually every country in the world.

Interesting leap year facts

• 97 leap years per 400-year cycle. In any 400-year span, there are 97 leap years, not 100. The three skipped leap years are the century years not divisible by 400 (e.g., 1700, 1800, 1900, 2100).
• February 29 is a "leap day." People born on February 29 are called "leaplings" or "leapers." Their birthday falls on the calendar roughly once every four years, so many celebrate on February 28 or March 1 in non-leap years.
• Leap years affect more than calendars. They're relevant in accounting (fiscal years), software (date handling), and astronomy (solar observations).
• The 400-year cycle contains 146,097 days. That's 97 leap years × 366 days + 303 non-leap years × 365 days = 146,097 days, which averages to 365.2425 days per year — extremely close to the tropical year of 365.2422 days.
• Not every calendar uses leap years. The Islamic calendar is purely lunar and does not use leap days. The Hebrew calendar uses a different system that adds an entire leap month rather than a leap day.

Common mistakes about leap years

• "Every 4 years is a leap year." Mostly true, but not always. Century years not divisible by 400 are not leap years. 1900, 2100, and 2200 are not leap years, even though they're divisible by 4.
• "2000 was not a leap year." It was. 2000 is divisible by 400, so it's a leap year. This was a rare "century leap year" that only happens once every 400 years.
• "Leap years have 366 days and no leap years have 365." True — but the difference is exactly one day: February 29. All other months keep their usual length.
• "Adding a day every 4 years exactly matches the solar year." It doesn't. The Julian calendar used this rule and drifted by about 1 day every 128 years. The Gregorian calendar's century rule corrects that drift.

Frequently asked questions